Cyclic uterine differentiation is being investigated through electron microscopy primarily in the albino rat and human being and with some attention to marsupials (Australian bandicoots and North American opossum). Postpartum regression in the rat uterus is being analyzed in terms of the effect that experimental variation of hormonal levels has on the normal differentiation of cellular populations, particularly those of the stroma. The effects of several estrogens and estrogen antagonists on the ultrastructural differentiation of the tissue compartments of the immature rat uterus are being compared. The normal differentiation of the human endometrium during the menstrual cycle is being analyzed ultrastructurally and cytochemically with particular emphasis on cellular mechanisms involved in the regression and subsequent regeneration of the endometrium. In addition, the ultrastructural features of the early gravid state of the human endometrium are being described. Current emphasis on the marsupial uterus centers on delineation of the fine structure of the maternal luminal epithelium of the Australian bandicoots (Marsupialia: Peramelidae) which forms multinucleate cells (homokaryons) during the luteal phase; in late gestation these homokaryons become heterokaryons only at the chorioallantoic placental site. BIBLIOGRAPHIC REFERENCES: Padykula, H.A. and J.M. Taylor: Cellular Mechanisms Involved in Cyclic Stromal Renewal of the Uterus. I. The Opossum, Didelphis Virginiana. Anat. Rec. 184:5-26, Jan., 1976. Padykula, H.A. and A.G. Campbell: Cellular Mechanisms Involved in Cyclic Stromal Renewal of the Uterus. II. The Albino Rat. Anat. Rec. 184:27-48, Jan., 1976.